Georeferencing

Georeferenced data is spatial data that is referenced to a location on the earth's surface. To do this, common frames of reference and coordinate systems have been set up.

Coordinate systems

It is clear that points and features on, above and below the earth's surface have position. To express that position, you need to choose an appropriate coordinate system so that positions can be defined within it. Such a system must obviously give each point a different coordinate.

There are thousands of coordinate systems in use throughout the world. It is possible for a user to invent their own and, indeed, this is often done for large engineering works. The disadvantage is that points outside the area of the bespoke system cannot be coordinated within it and the relationship between the bespoke and other systems is hard to define.

Methods of referencing data

There are two fundamental methods of referencing spatial data:

The first – known as the geo-centric system – uses a 3-D coordinate system with the centre of the earth acting as the origin of the three axes. This method is universally used in scientific applications, but it is user unfriendly when applied to points on the earth's surface. This is because the axes are made parallel to the spin axes of the earth rather than north (or some other arbitrary direction). The system can be expressed in two ways: either as a 3-D Cartesian coordinate of the form x,y,z; or as longitude (λ), latitude (φ) and height (H) above a known reference surface – the ellipsoid. It is important to note that the x and y do NOT refer to east and west or north and south.

The second, more common, system is the projection. This takes the 3-D coordinates and expresses them as a plane plus height above it. In other words, it flattens out the curved earth in a small region to a flat surface. Coordinates on the plane tell us where a particular point within the projection is being used and are measured as distances, east and north from the starting point or origin.

To avoid errors, the extent of the projection is usually limited to a small part of the earth's surface. Choice of projection is dependent on the area of the world being considered.

Different projections have different properties. Spatial data users choose the projection that provides the least distortion – in distance, direction, scale, area and so on – within the region being considered.

Bear in mind that there are dozens of different types of projection and each can have hundreds of different definitions depending on where they are used. In Great Britain the chosen projection is known as the National Grid.

The third dimension: height

Height can be expressed in two ways:

The first, and by far the most obvious, is to measure heights from sea level. However, this can be difficult as the sea level is irregular and constantly changing, making measurements of height inland both complex and expensive.

The second was created specifically because of these drawbacks. Scientists invented a more regular surface called an ellipsoid, which approximates to sea level. However, because the shape of sea level is complex, hundreds of different ellipsoids are required depending on the area of earth being modelled. Within Great Britain the ellipsoid of choice is know as Airy 1830 and this is used for the National Grid projection.

Global, regional and national systems

Many national mapping agencies have defined local referencing systems to meet their needs. In Britain, for example, we have the National Grid. These are perfectly acceptable when working within a country, but problems exist in drawing up multinational data unless there is cross-border convergence. In Europe local coordinate projections are often referred to the European Terrestrial Reference System 1989 (EtrS89) because it is fixed at a point in time and well defined.

Probably the most common global coordinate system in use today is the GPS-based World Geodetic System 1984 (WGS84). This is fixed to points on the earth's surface which move over time because of changes in the earth's crust. A significant global projection system is Universal Transverse Mercator (UTM). This is a defined set of projections that cover the whole world and allow countries to share spatial data more easily.

Once the digital data has been georeferenced, you can display the information in an infinite number of ways.